The suEXEC feature --
introduced in Apache 1.2 -- provides Apache users the ability
to run CGI and SSI programs
under user IDs different from the user ID of the calling
web-server. Normally, when a CGI or SSI program executes, it
runs as the same user who is running the web server.

Used properly, this feature can reduce
considerably the security risks involved with allowing users to
develop and run private CGI or SSI programs. However, if suEXEC
is improperly configured, it can cause any number of problems
and possibly create new holes in your computer's security. If
you aren't familiar with managing setuid root programs and the
security issues they present, we highly recommend that you not
consider using suEXEC.

Before jumping head-first into this document,
you should be aware of the assumptions made on the part of the
Apache Group and this document.

First, it is assumed that you are using a UNIX
derivate operating system that is capable of
setuid and setgid operations.
All command examples are given in this regard. Other platforms,
if they are capable of supporting suEXEC, may differ in their
configuration.

Second, it is assumed you are familiar with
some basic concepts of your computer's security and its
administration. This involves an understanding of
setuid/setgid operations and the various
effects they may have on your system and its level of
security.

Third, it is assumed that you are using an
unmodified version of suEXEC code. All code
for suEXEC has been carefully scrutinized and tested by the
developers as well as numerous beta testers. Every precaution
has been taken to ensure a simple yet solidly safe base of
code. Altering this code can cause unexpected problems and new
security risks. It is highly recommended that
you do not alter the suEXEC code unless you are well versed in
the particulars of security programming and are willing to share
your work with the Apache Group for consideration.

Fourth, and last, it has been the decision of
the Apache Group to NOT make suEXEC part of
the default installation of Apache. To this end, suEXEC
configuration requires careful attention to details from the
administrator. After due consideration has been given to the
various settings for suEXEC, the administrator may install
suEXEC through normal installation methods. The values for
these settings need to be carefully determined and specified by
the administrator to properly maintain system security during
the use of suEXEC functionality. It is through this detailed
process that the Apache Group hopes to limit suEXEC
installation only to those who are careful and determined
enough to use it.

Before we begin configuring and installing
suEXEC, we will first discuss the security model you are about
to implement. By doing so, you may better understand what
exactly is going on inside suEXEC and what precautions are
taken to ensure your system's security.

suEXEC is based on a setuid
"wrapper" program that is called by the main Apache web server.
This wrapper is called when an HTTP request is made for a CGI
or SSI program that the administrator has designated to run as
a userid other than that of the main server. When such a
request is made, Apache provides the suEXEC wrapper with the
program's name and the user and group IDs under which the
program is to execute.

The wrapper then employs the following process
to determine success or failure -- if any one of these
conditions fail, the program logs the failure and exits with an
error, otherwise it will continue:

Was the wrapper called with the proper number of
arguments?

The wrapper will only execute if it is given the proper
number of arguments. The proper argument format is known
to the Apache web server. If the wrapper is not receiving
the proper number of arguments, it is either being
hacked, or there is something wrong with the suEXEC
portion of your Apache binary.

Is the user executing this wrapper a valid user of
this system?

This is to ensure that the user executing the wrapper is
truly a user of the system.

Is this valid user allowed to run the
wrapper?

Is this user the user allowed to run this wrapper? Only
one user (the Apache user) is allowed to execute this
program.

Does the target program have an unsafe hierarchical
reference?

Does the target program contain a leading '/' or have a
'..' backreference? These are not allowed; the target
program must reside within the Apache webspace.

Is the target user name valid?

Does the target user exist?

Is the target group name valid?

Does the target group exist?

Is the target user NOT superuser?

Presently, suEXEC does not allow 'root' to execute
CGI/SSI programs.

Is the target userid ABOVE the minimum ID
number?

The minimum user ID number is specified during
configuration. This allows you to set the lowest possible
userid that will be allowed to execute CGI/SSI programs.
This is useful to block out "system" accounts.

Is the target group NOT the superuser
group?

Presently, suEXEC does not allow the 'root' group to
execute CGI/SSI programs.

Is the target groupid ABOVE the minimum ID
number?

The minimum group ID number is specified during
configuration. This allows you to set the lowest possible
groupid that will be allowed to execute CGI/SSI programs.
This is useful to block out "system" groups.

Can the wrapper successfully become the target user
and group?

Here is where the program becomes the target user and
group via setuid and setgid calls. The group access list
is also initialized with all of the groups of which the
user is a member.

Does the directory in which the program resides
exist?

If it doesn't exist, it can't very well contain files.

Is the directory within the Apache
webspace?

If the request is for a regular portion of the server, is
the requested directory within the server's document
root? If the request is for a UserDir, is the requested
directory within the user's document root?

Is the directory NOT writable by anyone
else?

We don't want to open up the directory to others; only
the owner user may be able to alter this directories
contents.

Does the target program exist?

If it doesn't exists, it can't very well be executed.

Is the target program NOT writable by
anyone else?

We don't want to give anyone other than the owner the
ability to change the program.

Is the target program NOT setuid or
setgid?

We do not want to execute programs that will then change
our UID/GID again.

Is the target user/group the same as the program's
user/group?

Is the user the owner of the file?

Can we successfully clean the process environment
to ensure safe operations?

suEXEC cleans the process' environment by establishing a
safe execution PATH (defined during configuration), as
well as only passing through those variables whose names
are listed in the safe environment list (also created
during configuration).

Can we successfully become the target program and
execute?

Here is where suEXEC ends and the target program begins.

This is the standard operation of the
suEXEC wrapper's security model. It is somewhat stringent and
can impose new limitations and guidelines for CGI/SSI design,
but it was developed carefully step-by-step with security in
mind.

For more information as to how this security
model can limit your possibilities in regards to server
configuration, as well as what security risks can be avoided
with a proper suEXEC setup, see the "Beware the Jabberwock" section of this
document.

Here's where we begin the fun. If you use
Apache 1.2 or prefer to configure Apache 1.3 with the
"src/Configure" script you have to edit the suEXEC
header file and install the binary in its proper location
manually. This procedure is described in an extra document. The following
sections describe the configuration and installation for Apache
1.3 with the AutoConf-style interface (APACI).

APACI's suEXEC configuration
options

--enable-suexec

This option enables the suEXEC feature which is never
installed or activated by default. At least one
--suexec-xxxxx option has to be provided together with the
--enable-suexec option to let APACI accept your request for
using the suEXEC feature.

--suexec-caller=UID

The username under which
Apache normally runs. This is the only user allowed to
execute this program.

--suexec-docroot=DIR

Define as the DocumentRoot set for Apache. This will be
the only hierarchy (aside from UserDirs) that can be used for
suEXEC behavior. The default directory is the --datadir value
with the suffix "/htdocs", e.g. if you configure
with "--datadir=/home/apache" the directory
"/home/apache/htdocs" is used as document root for the suEXEC
wrapper.

--suexec-logfile=FILE

This defines the filename to which all suEXEC
transactions and errors are logged (useful for auditing and
debugging purposes). By default the logfile is named
"suexec_log" and located in your standard logfile directory
(--logfiledir).

--suexec-userdir=DIR

Define to be the subdirectory under users' home
directories where suEXEC access should be allowed. All
executables under this directory will be executable by suEXEC
as the user so they should be "safe" programs. If you are
using a "simple" UserDir directive (ie. one without a "*" in
it) this should be set to the same value. suEXEC will not
work properly in cases where the UserDir directive points to
a location that is not the same as the user's home directory
as referenced in the passwd file. Default value is
"public_html".
If you have virtual hosts with a different UserDir for each,
you will need to define them to all reside in one parent
directory; then name that parent directory here. If
this is not defined properly, "~userdir" cgi requests will
not work!

--suexec-uidmin=UID

Define this as the lowest UID allowed to be a target user
for suEXEC. For most systems, 500 or 100 is common. Default
value is 100.

--suexec-gidmin=GID

Define this as the lowest GID allowed to be a target
group for suEXEC. For most systems, 100 is common and
therefore used as default value.

Compiling and installing the suEXEC
wrapper
If you have enabled the suEXEC feature with the
--enable-suexec option the suexec binary (together with Apache
itself) is automatically built if you execute the command
"make".
After all components have been built you can execute the
command "make install" to install them. The binary image
"suexec" is installed in the directory defined by the --sbindir
option. Default location is
"/usr/local/apache/sbin/suexec".
Please note that you need root
privileges for the installation step. In order
for the wrapper to set the user ID, it must be installed as
owner root and must have the setuserid
execution bit set for file modes.

Upon startup of Apache, it looks for the file
"suexec" in the "sbin" directory (default is
"/usr/local/apache/sbin/suexec"). If Apache finds a properly
configured suEXEC wrapper, it will print the following message
to the error log:

[notice] suEXEC mechanism enabled (wrapper: /path/to/suexec)

If you don't see this message at server startup, the server
is most likely not finding the wrapper program where it expects
it, or the executable is not installed setuid
root.
If you want to enable the suEXEC mechanism for the first time
and an Apache server is already running you must kill and
restart Apache. Restarting it with a simple HUP or USR1 signal
will not be enough.
If you want to disable suEXEC you should kill and restart
Apache after you have removed the "suexec" file.

Virtual Hosts:
One way to use the suEXEC wrapper is through the User and Group directives in VirtualHost definitions.
By setting these directives to values different from the main
server user ID, all requests for CGI resources will be executed
as the User and Group defined for that
<VirtualHost>. If only one or neither of
these directives are specified for a
<VirtualHost> then the main server userid is
assumed.

User directories:
The suEXEC wrapper can also be used to execute CGI programs as
the user to which the request is being directed. This is
accomplished by using the "~"
character prefixing the user ID for whom execution is desired.
The only requirement needed for this feature to work is for CGI
execution to be enabled for the user and that the script must
meet the scrutiny of the security checks
above.

The suEXEC wrapper will write log information
to the file defined with the --suexec-logfile option as
indicated above. If you feel you have configured and installed
the wrapper properly, have a look at this log and the error_log
for the server to see where you may have gone astray.

NOTE! This section may not be
complete. For the latest revision of this section of the
documentation, see the Apache Group's Online
Documentation version.

There are a few points of interest regarding
the wrapper that can cause limitations on server setup. Please
review these before submitting any "bugs" regarding suEXEC.

suEXEC Points Of Interest

Hierarchy limitations

For security and efficiency reasons, all suexec requests
must remain within either a top-level document root for
virtual host requests, or one top-level personal document
root for userdir requests. For example, if you have four
VirtualHosts configured, you would need to structure all
of your VHosts' document roots off of one main Apache
document hierarchy to take advantage of suEXEC for
VirtualHosts. (Example forthcoming.)

suEXEC's PATH environment variable

This can be a dangerous thing to change. Make certain
every path you include in this define is a
trusted directory. You don't want to
open people up to having someone from across the world
running a trojan horse on them.

Altering the suEXEC code

Again, this can cause Big Trouble if you
try this without knowing what you are doing. Stay away
from it if at all possible.